UPDATE: The version was not approved because to make it work, I basically used the equivalent of a proxy removing the needed CSP headers. AMO Reviewers obviously were not happy. I might need to write a client from scratch to solve this.

Thanks for letting me know of this bug. I have just updated the add-on and think I have fixed it. At least I can register and use mamot.fr without problems as can be see on the screen shot at http://take.ms/COFLq

The new version is numbered 1.1, it is on the queue waiting for approval. You can follow me on Mastodon at soapdog@toot.cafe, I will announce the new version there once it is approved.